This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Men and women undergo a progressive reduction in lean muscle mass (sarcopenia) with advancing age regardless of their level of physical activity. Androgen stimulates skeletal muscle mass in men. This increase in muscle mass occurs through an improvement in net protein balance- determined by either an increase in muscle protein synthesis or an inhibition of muscle protein breakdown. Cycling testosterone could provide a more consistent increase in muscle strength while reducing the exposure to androgens that would limit potential side effects from long- term exposure. The aims of this study are to: 1) determine the effect of continuous and cycled testosterone administration on muscle myofibrillar and mitochondrial protein synthesis, 2) determine the effect of cycled and continuous testosterone administration on skeletal muscle single fiber function, and 3) determine the effect of cycled and continuous testosterone administration on skeletal muscle single fiber function. Normal healthy older men ages 65-85 will be studied. This clinical protocol is designed to compare specific outcomes in older men between three groups- placebo, men receiving testosterone enanthate (TE, 100 mg) weekly by intramuscular injection for 5 months, and men receiving TE weekly for 1 month alternating with weekly placebo injections for 1 month for a total of 5 months. An additional group receiving TE on a week-on, week-off basis was initiated, however this was halted due to financial constraints that would not allow the study of additional subjects beyond those originally planned.